Position a rack in the center of the oven and preheat to 350. Place paper liners in muffin pan.

Prepare the cupcakes: Bring the sugar and 1 cup water to a boil in a saucepan, stirring until the sugar dissolves, about 3 minutes. Pour into a large bowl; add the chocolate and butter and let sit, stirring occasionally, until the chocolate is melted and the mixture has cooled slightly. Stir in the vanilla.

Sift together the flour, baking soda, baking powder and salt in a bowl.

Using a mixer, beat the eggs into the chocolate, then mix in the dry ingredients.

Divide the batter evenly between the prepared pans (about 1/4 cup batter per cupcake) and bake until a toothpick inserted in the center comes out clean, about 25 minutes. Cool in the pans for about 25 minutes, then transfer cupcakes to a rack to cool completely. (They will sink in the middle…they’ll puff up when filled later)

For the filling: Using a mixer, cream the butter until light and fluffy. Beat in 1/2 cup confectioners’ sugar. Add the vanilla and 1 tablespoon heavy cream; beat until smooth. Beat in the remaining 1/2 cup confectioners’ sugar and 2 tablespoons heavy cream in batches, alternating after each addition. Beat in the marshmallow creme; set aside or refrigerate.

For the ganache: Place the chocolate in a stainless-steel bowl. Heat the cream and 1 tablespoon butter until just boiling, then pour over the chocolate; let stand for 5 minutes. Whisk until smooth. Add 2 teaspoons vanilla; let stand until cool but still glossy and liquid.

Spoon the filling into a pastry bag with a medium star tip. Insert the tip into the center of each cupcake top; fill until the cupcake is heavier (do not overfill). (It’s OK if some of the filling peeks out, it will be covered by the ganache.) Spoon (I dipped) a little ganache on each cupcake and lightly spread with an offset spatula or a knife. Chill for at least 15 minutes.

Meanwhile, prepare the icing: Using a mixer, beat the remaining 1 stick butter, 1/4 teaspoon vanilla, the milk and confectioners’ sugar until smooth, adding more milk if needed. Spoon into a pastry bag with a small tip (I used a plastic bag); pipe onto the cupcakes to decorate.